Various processes have been proposed in the past for treating waste oxides from the steel mill in order to recover iron values and separate contaminant constituents such as zinc and lead oxides and the like. One such prior art process is disclosed in U.S. Pat. No. 5,601,631. The process of this patent includes the steps of forming a dry mixture of metal oxide waste and a carbonaceous material, agglomerating the mixture, as by briquetting, to form green compacts, firing the compacts in a rotary hearth furnace at an effective hearth temperature of about 2100.degree. F.-2450.degree. F. for about five to twelve minutes. The carbonaceous material is one containing a high volatile matter content which is mobilized to bond the dry mixture and form the green compacts. In the preferred embodiment, the carbonaceous material is coal. As disclosed in the patent, metal oxides of iron, lead, cadmium and zinc are reduced at relatively low CO/CO.sub.2 ratios of between 2-5, most preferably about 3. The patent claims over 99% removal of zinc, cadmium and lead oxide.
U.S. Pat. No. 4,780,135 discloses a method of processing waste metal oxides by combining the dust with carbon, such as coke breeze, coal dust, charcoal or any other suitable carbon source, and pelletizing the mixture. The pellets are fired in a rotary hearth furnace at temperatures ranging between 1100.degree. F.-2000.degree. F. for about eight to twenty minutes.
U.S. Pat. No. 3,836,353 discloses a method of recovering iron and oxide impurities from steel furnace dust in which the dust first is mixed with finally divided coke and then pelletized. The pellets are processed in a rotary hearth furnace through a drying zone (500.degree. F.-600.degree. F.), a heating zone (1500.degree. F.-1600.degree. F.), a zone heated to about 2000.degree. F. where contaminants such as zinc are evaporized and removed, and a reoxidizing and hardening zone (2000.degree. F.-2450.degree. F.) for about seven to fifteen minutes.
As generally described above, the prior art processes of recovering iron and separating oxide impurities from steel mill waste products generally involve mixing the waste oxides with a carbonaceous material, agglomerating the mixture by pelletizing or briquetting to form green compacts, and firing the compacts in a rotary hearth furnace at temperatures up to about 2500.degree. F. in order to metallize the iron values and volatilize the contaminant oxides such as zinc and lead. While some of these prior processes have found some acceptance, none have been completely successful because of certain difficulties addressed by the process of the present invention. For example, pelletizing requires wetting of the blend with up to about 12% moisture and then shaping into round pellets. Upon drying, the pellets develop a relatively tight skin which impedes fast outward movement of reaction gases. Unless predried and heated slowly, which are costly impediments to productivity, the pellets exfoliate. Large pellets of about 1/2 inch have been found to "pop" even when predried to less than 1% moisture due to build-up of excessive internal pressure resulting from contained oil, CO, etc. This disintegration of pellets results in a carry-over of iron oxide particles into the gas stream and into the bag house catch, thereby adversely affecting zinc quality and reducing the recoverable iron values for further steel making operations. The use of small pellets in an attempt to avoid disintegration results in multi-layer packing on the hearth which retards heat transfer and lowers the extent of metallization and zinc removal.
Binder aids, such as bentonite and the like, have been used to make pellets with sufficient green strength to withstand entry into the rotary hearth furnace. The use of bentonite has a disadvantage of introducing materials that promote melting and slagging of oxides in and on the pellets. More particularly, bentonite adds silicates and alkali metals to the already alkali oxide rich waste oxides, thereby lowering the temperature at which these materials melt and form a slag. The slagging of the pellets restrict the maximum operating temperature and, hence, the productivity of the furnace.
While briquettes are less prone to exfoliation than pellets and can achieve faster metallization, certain disadvantages have been experienced in their use. Conventionally made briquettes have exhibited slagging and a high degree of reoxidation in the microstructure. As in the case of pellets, slagging results from melting undesirable oxides at the temperatures needed to achieve optimum metallization. Slagging ties up iron oxide making it more difficult to reduce. Reoxidation decreases the degree of metallization.
In an effort to minimize carbon depletion in briquettes which results in reoxidization of the iron, it has been proposed to combine the waste oxides with coal prior to briquetting. The use of coal is not completely effective because it oxidizes rapidly under furnace conditions to result in a carbon depleted zone on the outside of the briquette. When carbon has been eliminated from the surface, reoxidation can begin even if metallization is not complete in the interior, since carbon in the interior will not prevent reoxidation at the surface. Further, the volatile material introduced into briquettes by coal can cause the briquettes to crack during firing.